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Old 27th May 2008, 03:17 PM   #1
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Default Grounding: Earth and Center Tap

I need a sanity check folks. What would happen if I attach the center tap of the high voltage secondary of a power transformer to Earth ground? Pat on the back or smoke and flames? Should Earth be used to ground the chassis and the center tap for the circuit?
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Old 27th May 2008, 06:17 PM   #2
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You're still sane, at least in regards to this design. If you look at most classic tube amp designs, the center tap is grounded to the chassis, thus establishing it as the relative zero volt point. With modern design philosophy, I would suggest that instead of connecting it directly to your chassis earth, you use a NTC thermistor or a bridge rectifier between the transformer center tap and the earthing point. This will help keep hum off of your circuit zero volt path. Also, attach your earth safety wire of your power cord directly to your chassis, then bring all of your circuits grounds together at one star ground point near the point where you attached your safety wire.

Peace,

Dave
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Old 27th May 2008, 06:45 PM   #3
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I never thought about using a thermistor. Good idea. Any suggestions for an appropriate value? The transformer I'm using is 250-0-250 (500VCT). I see one that can do 180 ohms. Typically I've used 10k resistors for closing ground loops which are effective, but may not give the best results as far as safety is concerned.
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Old 27th May 2008, 07:02 PM   #4
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Hello labrat, I don't know of the top of my head what value thermistor to use. I got the general idea from one of Nelson Pass writeups about his ZEN amps. If you go to www.passdiy.com and take a look at the ZEN amp series, he shows using Thermistors. That's also where I got the idea of using a Bridge rectifier.
Basically you want some sort of device that keeps the circuit ground isolated from the earth(safety) ground in normal operation, but allows any excess current to flow to the safety ground when necessary. That's why a Bridge rectifier will also work.

Peace,

Dave
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Old 9th April 2012, 06:40 PM   #5
Legis is offline Legis  Finland
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Hello folks,

I have used center tapped 3kVA transformer as a balanced isolation transformer. Right now the secondary's center tap acts as a "artificial" ground for the equipment and it's not connected to the real ground.

This site (The Origin of Balanced Power) shows that the center tap could/should be connected to the ground and this got me thinking if I should ground the center tap.

What are the negative/positive sides of grounded versus lifted center tap in an isolation transformer?
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Old 10th April 2012, 12:44 AM   #6
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Floating systems are bad. Stray capacitance to ground can result in transient over voltages, well documented in a number of the IEEE color books (Red, Green, Emerald).

You will find any number of people in this forum use anecdotal 'evidence' to argue better performance when the secondary was left ungrounded. These are at best band-aids to systems having problems, and they justified floating the 120VAC source to mask the underlying problem.

NEC prohibits floating systems in residential use; the same people above will care less about the code. NEC compliant balanced systems are also grounded.

One of the most significant characteristics of a transformer is to allow the secondary to use a whole new grounding system, isolated from the primary. Why not use this huge benefit of galvanic isolation.
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Old 10th April 2012, 12:49 AM   #7
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In regards to the usage of an isolation transformer for an entire system, I would agree with zigzagflux, and it seems he has researched this pretty well.

I was only commenting with regards to the center tap of the secondary of a transformer being used to supply the power for inside of an amp or preamp. You should always provide a ground path for your system as a whole. Especially in regards to the issue of electrical connection of any metal enclosure(s) to AC. Potential shock hazard there, you need to make sure that your metal enclosure are safe and don't hurt anyone.

Peace,

Dave
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Old 10th April 2012, 08:45 AM   #8
Legis is offline Legis  Finland
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Thanks for the answers. I also found this: www.gearslutz.com/board/attachments/remote-possibilities-acoustic-music-location-recording/157192d1265705970-power-isolation-transformer-power.pdf

It states that grounding the CT (center tap) of the isolation transformer gives fully balanced system, but leaving the CT floating gives better safety compared to regular mains connection.

In any case I think I will ground the center tap today. Do I benefit from using bridge rectifier between the CT and the ground?
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Old 10th April 2012, 11:50 AM   #9
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That trick is sometimes used to offset circuit common within a piece of equipment when using unbalanced interconnects. It should never apply to an isolation transformer providing balanced or unbalanced power.

Disagree on a floating system being safer- one should ask themselves why the NEC (or any other peer reviewed engineering association) doesn't condone floating systems if they are supposedly safer. The NEC is written primarily to prevent fires and personal shock. Would they not pick the 'safest' solution ?
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Old 10th April 2012, 11:57 AM   #10
AndrewT is offline AndrewT  Scotland
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Why is there so much confusion when discussing Earth & Ground?

It is simple. Keep the Safety issue of PE quite separate from the Audio issue of Signal Returns. Don't mix them up by using ambiguous terms that can be swapped between Safety and Audio.

The Audio side does NOT need a connection to PE to operate well.

The Safety Earth is only there to protect users if a fault should develop. It is NOT there to make the Audio better.
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